The invention relates to the field of hard material powder and hard metal granules for producing hard metal coatings. These are dense materials that are also very hard. These materials are preferably applied in spherical form to tools, for example, boring tools and bore rods or others. This is to impart a high resistance to wear and a toughness to these tools and parts that diminishes the effects of abrasion and impact.
The invention relates in particular to spheroidized hard metal powders, which are also generally represented as MexMy powders or corresponding granules, which are applied for coating expendable parts, inter alia, by means of flame spraying, plasma spraying and related technologies. Me is defined as a metal and M is defined as a metalloid. The preformed powder is thus sprayed onto the surface to be coated, for example, in direct-current plasma.
According to a classic production process for a coating powder in the afore-mentioned sense, for example, the following process steps are executed: A basic mixture of hard metal powder (for example based on WC/W2C) is initially produced by mixing and grinding the components. This mixture is then converted to a largely homogeneous melt at about 3,000xc2x0 C. After cooling this melt, the fused-together hard metal is comminuted and screened. A fraction having a preset fine grain size is then rounded off by repeated heating (this may take place in a plasma) and used for coating the expendable parts after final cooling.
As can be seen easily, the known process is already very expensive due to the number of working steps. In addition, this process is intensive in terms of energy and cost, which is the result, inter alia, of the production of the high-temperature melt and the subsequent comminution of the hard material.
A method, which should simplify and shorten the above-mentioned process, is also already known from European Application 0 687 650. The hard material, for example, tungsten carbide, is thus melted in a crucible by means of a plasma flame. The use of a plasma flame provides a significant shortening of the melting time. After producing the hard material melt, the latter is passed in a defined melt stream to a rapidly rotating cooling disc. The cooling disc is rotated at very high speed and thus cooled, resulting in very fine hard material spheres. As a result of this process, hard material granules of a certain quality and having a certain structure are produced, on which influence may be effected only to a very limited extent. The need for novel hard metal coating powders that can be produced cost-effectively is significant.
The present invention is directed to overcoming one or more of the problems set forth above.
An aspect of the invention is to provide a good-flowing, thus separation-free powder or granules for hard material coating, in particular of expendable parts, by plasma spraying, and to develop a process for its production, which may be carried out in as few working steps as possible and is efficient in terms of cost and energy.
Another aspect of the invention is a process for producing spheroidized hard material powder is disclosed. This process includes the following process steps of producing a finely ground mixture of hard material, wherein the mixture of the hard material is selected so that under conditions of a high-frequency plasma, a reaction starts between constituents of the mixture of the hard material, and introducing the mixture of the hard material with a carrier gas stream into a working gas stream of a thermal, inductively coupled, high-frequency plasma and as a result of which the reaction occurs in one step with a formation of spheroidized hard metal particles.
The first step is the production of a mixture of hard material. The constituents of the hard material required for later coating or starting materials for this hard material, which are later reacted in the process with a reactive gas within the plasma, are thus mixed and finely ground, for example in an attrition mill.
The mixture may be used either directly in a suspension or additionally may be finely granulated, for example in spray drying with optional subsequent degassing. A suspension may also be produced from the hard material powder, for example, may be produced using a hydrocarbon which reacts with the powder components in the plasma.
The mixture thus produced in powder, granules or suspension form is then introduced into the working gas of a thermally, inductively coupled high-frequency plasma, hereinafter also referred to as xe2x80x9cICPxe2x80x9d, within a carrier gas stream. The above-given hard material mixture with the carrier gas stream is thus blown through the plasma arc of the high frequency plasma.
There are a number of ICP plasma systems known or available so that a description of a suitable apparatus is completely unnecessary.
The particles are spheroidized after passage through the plasma arc are then cooled in an additional quench gas stream at high speed below a recrystallization temperature and are collected behind the plasma. The quench gas stream is an additional cooling gas stream that is generally inert and supplied separately to the system.
The extremely compact process design is particularly advantageous in the invention. This is due to the fact that both the reaction of the components with one another, alloy formation as well as the spheroidization take place in one unitary step within the plasma. The separate step of melting the starting materials, and hence optionally, also the subsequent steps of comminution, screening, rounding-off of the melt product, are omitted. The course of the process is very much simplified and shortened. The process therefore operates in very efficient manner in terms of energy and cost.
A spherical hard material powder, which is homogeneous in its composition and shows good flow behavior when processing in coating application, is obtained. A xe2x80x9cspheroidized hard material powderxe2x80x9d is thus understood to mean a powder made from a hard material alloy having completely rounded-off particle edges.
The spheroidal granules obtained according to this Invention have the advantage of a structure which is more uniform compared to granules produced by other processes including the quality of the spherical shape for the individual particles. By using the process of this Invention, it is possible to produce the particle size distribution in relatively narrow and adjustably different size classes.
A hard material or a hard material alloy within the scope of the invention is understood to mean in the narrower sense a compound of the form MexMy, wherein Me is a metal and M is a metalloid (the formula should be understood so that different metals and metalloids may be combined). Specifically, xe2x80x9cmetallic hard materialsxe2x80x9d are therefore understood to mean chemical compounds of Transition Metals of the Subgroup IVa to VIa of the Periodic Table with the small atomic elements carbon, nitrogen, boron and silicon, that is the carbides, nitrides, carbonitrides, borides and silicides of the metals: titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten and mixtures thereof.
The alloy system (W2C)0.5+z(WC)0.5xe2x88x92z where (z less than 0.5) with high toughness and a high Vickers hardness of greater than 2,000 is preferred within the scope of this Invention.
The starting constituents for the basic mixture of hard metal powder may thus include, but is not limited to, one of the following groups:
a) W2C+WC;
b) WC+W;
c) W+C;
d) W+CnH2n+2; and
e) W+others.
In this case, W is tungsten, C is carbon and H is hydrogen. The starting materials may be present as metals or metal oxides or as preformed alloys between certain individual constituents.
To maintain the ICP plasma, working and enveloping gas stream is also required. A carrier gas stream is required in this gas stream for blowing in the basic mixture in powder, granule or suspension form, and for rapid cooling of the particles after the plasma, a so-called quenching gas stream is required.
In addition to a reaction between hard material starting materials, a reaction with the working and/or carrier gas may therefore also take place, provided it is not a gas which is inert with respect to the constituents of the basic mixture, for example a noble gas, preferably argon.
If a reactive gas is used, it may be selected, for example so that under the conditions of the plasma, it forms carbides with metals or metal oxides of the basic mixture constituents. Methane is preferred in this case. Also, if nitrides are formed, then nitrogen would be the preferred gas in this situation.
The reactions between the basic mixture constituents and the reactive gas may be shown, inter alia, by the following basic formulas:
aW+bCH4@c(W2C)0.5+z(WC)0.5xe2x88x92z+dH2;
aTi+bN2@cTiN;
and
aTa2O5+bCH4@c(TaC)x(Ta2C)y+dH2O.
In this case, W is tungsten, C is carbon, H is hydrogen, Ti is titanium, Ta is tantalum, O is oxygen, and N is nitrogen.
The thermal, inductively coupled high-frequency plasma is preferably operated at a temperature above 3,000xc2x0 C., also preferably above 4,000xc2x0 C. The high inductive field has a generally accelerating effect on the reaction rate and a positive effect on the reaction equilibrium during the formation of W2C/WC.
The basic mixture reacted and blown through the HF plasma is preferably quenched at cooling rates greater than 104 K/s.